The present invention relates to a magnetic drive device for magnetically rotating two rotors independently of each other, for example, for use in semiconductor production apparatus for handling wafers in a vacuum.
Drive devices of the type mentioned are known which comprise an outer rotor drivingly rotatable by noncontact drive means, e.g., a magnetic coupling, and an inner rotor drivingly rotatable by a motor mounted on the outer rotor.
With the conventional drive device, the motor or like drive means for driving the inner rotor is provided on the outer rotor, so that the drive means for the two rotors can not be separated from the rotors completely, failing to serve as noncontact means.
Accordingly, for example in the case where the two rotors are to be placed in a vacuum separated off by a vacuum shield for use in a semiconductor production apparatus or the like, the inner rotor drive means on the outer rotor must also be placed in the vacuum. This leads to various disadvantages.
Further with the drive device described, there arises the necessity of moving the two rotors axially thereof.
Drive devices are generally known for magnetically driving a rotor in rotation and also axially thereof without contact. The device comprises a movable housing which is disposed around the rotor movably axially of the rotor and provided with a magnetic bearing for holding the rotor with respect to the radial direction and the axial direction and with a drive portion of a magnetic coupling for transmitting a torque to the rotor.
The rotor is rotatable by rotating the drive portion of the magnetic coupling, and is movable axially thereof by axially moving the housing owing to the axial holding force of the magnetic bearing.
In the case where the rotor is placed in a vacuum separated off by a vacuum shield, a hollow cylindrical wall portion of the shield is provided between the rotor and the housing, whereas the following problems arise if the drive device is used for driving the rotor which is disposed inside the wall portion.
The magnetic bearing generally has a small air gap and is movable along the cylindrical wall portion, so that the wall portion must be given a reduced thickness and prepared with high precision. This poses problems with respect to quantity production of the drive device and the quality thereof.
Accordingly, the magnetic bearing is to be actually used with a large air gap, whereas this reduces the holding force of the bearing especially in the axial direction, consequently necessitating an increased control current and therefore a magnetic bearing of larger size to obtain a sufficient axial holding force.